A variable valve apparatus that changes a valve characteristic of an intake valve according to an engine operating state is known, as is described in Patent Document 1, for example.
The variable valve apparatus described in Patent Document 1 includes a variable mechanism portion (intermediate driving mechanism in the document) that changes the valve characteristic of the intake valve, a control shaft that operates the variable mechanism portion, a cam that moves the control shaft in an axial direction, and a motor that rotates the cam. Also, variable control of the valve characteristic is performed by controlling the rotation phase of the cam.
Here, force in an axial direction (hereinafter, referred to as axial force) is applied to the control shaft as a result of reaction force of a valve spring that urges the intake valve, and a rotational moment is generated in the cam by this axial force acting on a cam face. Therefore, with a mechanism that moves the control shaft using a cam, when the motor stops being driven, the cam rotates and the valve characteristic changes. In order to inhibit this kind of change in the valve characteristic, force against the rotational moment must be generated by the motor. Therefore, when using an electric motor as the motor for rotating the cam, a holding current must be supplied to the motor in order to inhibit a change in the valve characteristic.
Therefore, with the apparatus described in Patent Document 1, a section where a displacement amount of the controls shaft changes, and a section where this displacement amount is constant, are provided on the cam face of the cam. That is, a changing region where the valve characteristic changes by the displacement amount of the control shaft changing, and a holding region where the displacement amount of the control shaft is constant and does not change, such that the valve characteristic is kept at a constant value, are provided on the cam face.
When the cam face in this holding region, i.e., the cam face where the distance from the rotational center of the cam is constant, is used, the rotational moment caused by the axial force is inhibited from being generated, so rotation of the cam is suppressed. Therefore, even if the motor stops being driven, the valve characteristic is maintained at a valve characteristic corresponding to the holding region. Thus, a holding current such as that described above can also be reduced, for example. Also, if the rotation phase of the cam falls within the range of the holding region, the valve characteristic is able to be kept at a constant value, so there is no need for highly precise phase control as motor control when keeping the valve characteristic at the constant value, unlike with a variable valve apparatus that is able to continuously change the valve characteristic.
Also, if a plurality of the holding regions described above are provided on the cam face, a plurality of difference valve characteristics are able to be maintained, and the valve characteristics are able to be changed in multiple stages. That is, a variable valve apparatus capable of changing a valve characteristic in multiple stages by selecting a valve characteristic from among a preset plurality of valve characteristics (hereinafter, referred to as a multi-stage variable valve apparatus) is able to be obtained.